1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, an imaging device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a negative electrode for a secondary battery and a secondary battery.
2. Description of the Related Art
As portable information terminals typified by smart phones and tablet terminals and mobile devices such as laptop personal computers and portable game machines became popular, secondary batteries having high energy density which are small and lightweight became urgently necessary, and some of them have been developed in recent years.
It is important to increase capacity density of a negative electrode in order to increase the energy density of a secondary battery. Graphite, which is widely used as a negative electrode active material of a lithium-ion secondary battery, has a theoretical density of capacity of 372 mAh/g, and already has 360 mAh/g or more capacity density which is close to the theoretical capacity.
To further increase the energy density of a secondary battery, a negative electrode active material with higher theoretical density of capacity has been examined, and silicon (Si), tin (Sn), germanium (Ge), and gallium (Ga) which are alloyed with lithium, an oxide thereof, an alloy thereof, and the like have attracted attention.
For example, Patent Document 1 and Patent Document 2 disclose a secondary battery in which a material containing silicon is used for a negative electrode active material.